Some prior art xerographic devices use an adjustable cover on a transfer Di-chorotron, or “Dicor,” to eliminate paper edge ghost (PEG) defects in output. PEG defects are observed as a difference in halftone densities after a change in media size, resulting from trapped positive charge in directly exposed areas of the photoreceptor. For example, such ghosts can be caused through use of the same size of paper for a given number of cycles, then switching to a different size of paper that at least partially exposes the portions of the photoreceptor that are not as fatigued from use. The newly exposed portions of the photoreceptor thus respond to the xerographic process differently, producing a paper edge ghost.
The cover blocks the transfer current to the photoreceptor outside the paper width area. The transfer power supply works to control a constant transfer current. Unfortunately, as the cover closes off a portion of the Dicor, the current density supplied to the paper area increases, which can cause variations in output quality. To maintain a constant transfer current, the operator must make adjustments to the transfer current settings every time paper width changes, which is cumbersome. Additionally, the use of an inboard transfer cover as currently configured, while effective, is tedious from the customer perspective, requiring removal of the transfer device, repositioning the cover and manually resetting the transfer current by entering the media type and paper width, a further complication that will grow as the media list expands over time.
A proposed solution to eliminate operator adjustment of the transfer current settings is to incorporate these settings into the media library stored within the xerographic machine. In this manner, the xerographic machine's controller would look up the proper transfer current settings for a given type/size of media. However, this would require many more entries for all the combinations of media type and width customers might employ. Customers have complained because of the complexity and tediousness of current operation, and making such operation more complex is likely to be further dissatisfying to customers.
Embodiments modify the sliding transfer Dicor cover by adding a conductive electrode and connecting the electrode to a grounded external impedance that simulates a photoreceptor impedance. With such modifications, the current density captured by the electroded sliding transfer Dicor cover is the same as in the media area. This maintains a constant media current density as the cover occludes different widths of the Dicor. The external equivalent circuit simulates the impedance of paper on photoreceptor making the portion of the photoreceptor that has no media, yet faces the covered Dicor, “look more like” the paper covered area. This enables constant transfer current to the media independent of the extent of coverage of the wire by the sliding electroded transfer cover. As a result, the sliding cover can be moved anywhere within the required range without resetting the power supply transfer current. Embodiments thus eliminate the need for having an operator change transfer current settings whenever media width changes. Embodiments provide for different combinations of conductive electrode geometry on the sliding cover and/or the impedance of a passive external grounded circuit to create the impedance required to simulate a photoreceptor in the covered area.
Mechanical constraints prohibit a simple grounded electrode from being at or very near the photoreceptor surface. Because the inside of the cover is closer to the Dicor wire than the photoreceptor surface, electric fields are higher and arcing might occur. Embodiments can employ an AC and/or DC voltage bias on the electrode to reduce or eliminate arcing. Any grounded external impedance connected to the electrode will result in a passive AC and/or DC electrode voltage bias generated by the voltage drop in the external impedance from the electrode current. The passive impedance of embodiments can be as simple as a resistor or can include back-to-back Zener diodes and a series resistor. This impedance, and in the case of embodiments with Zener diodes the impedance is non-linear, will allow the electrode to partially follow the high voltage wire AC and to reduce the risk of arcing from the high voltage wire to the shield electrode. The current collected on the shield electrode is measured by the power supply as a transfer current since it is ultimately passed to ground, allowing the paper current density to remain constant as the sliding cover changes position.